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Abstract

We demonstrate the use of holographic optical tweezers for trapping and manipulating
silicon nanomembranes. These macroscopic free-standing sheets of single-crystalline
silicon are attractive for use in next-generation flexible electronics. We achieve
three-dimensional control by attaching a functionalized silica bead to the silicon
surface, enabling non-contact trapping and manipulation of planar structures with
high aspect ratios (high lateral size to thickness). Using as few as one trap and
trapping powers as low as several hundred milliwatts, silicon nanomembranes can be
rotated and translated in a solution over large distances.

Keywords:

Introduction

Silicon nanomembranes are flexible, single-crystalline sheets with thicknesses ranging
from less than ten up to several hundred nanometers [1,2]. These materials are extremely attractive for use in fast-flexible-electronic, optoelectronic,
and nanophotonic applications. This broad potential derives from the unique properties
imparted by the membranes' thinness relative to silicon wafers, including robustness,
flexibility, and bondability. The structures can also be strain engineered to enhance
individual electronic and mechanical properties or to produce unique tubular and helical
nanostructures [2-6]. Successful integration of these structures into next-generation devices will require
new paradigms for their assembly. The most promising methods for transferring and
manipulating silicon nanomembranes to date include wet transfer (whereby nanomembranes
are moved from the original substrate in a solution via adhesive attachment to a new
host), dry transfer, and stamp printing processes [7-9] As nanomembranes are made thinner and thus become more difficult to handle, mechanical
means of manipulation are limited in their precision with regards to controllably
placing individual membranes.

Holographic optical trapping [10-13] offers a promising new approach for manipulating silicon nanomembranes with a high
degree of accuracy and precision that may circumvent some of the above issues. Optical
tweezers use a single, tightly focused beam of light to manipulate micro- and nanoscale
objects in three dimensions. The technique enables precise positional control in a
non-contact and non-invasive fashion without damage to the trapped object [14]. Holographic optical trapping uses an array of traps to extend these same capabilities
to multiple points in space. Each trap can be independently and dynamically controlled
in real time. Although holographic optical tweezers have been used to manipulate microspheres,
nanowires, and arbitrarily shaped biological molecules [15-18] application to two-dimensional (planar) geometries has been limited to objects with
low aspect ratios and with dimensions less than approximately 5-10 × 5-10 μm [19,20] The nanomembranes used for this work have thicknesses of 220 nm and in-plane dimensions
of 50 × 50 μm, giving edge length-to-thickness aspect ratios of over 200. We believe
this is the first time high-aspect-ratio planar materials have been successfully manipulated
using non-contact techniques.

Most recent work on trapping and manipulating semiconductor materials has focused
on one-dimensional nanowires [21,22], where the large index of refraction mismatches with the fluid medium complicates
direct manipulation with the beam. Nanowires also tend to align along the axis of
the laser and can be completely ejected from the trap, because of the overwhelming
scattering force. This problem was recently circumvented for high-index vanadium oxide
nanowires [23] using silica beads as a handle, the optical trapping of which has been well described
in the literature [24]. The attachment of beads is commonly used when working with biological materials,
where the arbitrary shape of the molecule or damage from laser heating can preclude
trapping [25,26]. Wider application to directed assembly is limited, however, because removal of the
handle has remained an issue: the manipulated object needed to be laser cut or damaged
to achieve detachment or in other cases the covalently bonded bead could not be detached
at all.

We present here a significant step forward, extending this initial work to the silicon
nanomembranes described above. By attaching a single bead to the edge of a membrane,
we achieve full three-dimensional control of large-area planar objects using optical
tweezers. We furthermore succeed in reversible detachment of the handle bead without
damage to the membrane, providing a substantial advantage over previous work. These
capabilities will enable new, flexible routes for assembly of a variety of two-dimensional
thin sheets of direct relevance to semiconductor, biotechnology, and sensor technologies.

Results and discussion

Attachment of silica microsphere handles is shown in Figure 1. The nanomembranes were dispersed in a de-ionized water solution with trace amounts
of isopropyl alcohol. 3 mL of this suspension was pipetted into an open sample well
with a glass coverslip bottom. The membranes submerge to an equilibrium distance several
hundred micrometers below the surface of the liquid. After a suitable membrane was
identified, commercially available 10-μm diameter silica beads with an amine terminated
(-NH2) surface functionalization (Sicastar®, Micromod Partikeltechnologie GmbH, Rostock-Warnemünde, Germany) were attached to
the membrane using the optical trapping system. The functionalized microspheres, dispersed
in de-ionized water, were added to the nanomembrane suspension. A single suspended
bead was stably trapped at a power of approximately 300 mW and the submerged nanomembrane
moved towards it using a Prior ProScan II (Rockland, MA, USA) motorized microscope
stage. The bead was directed to near the desired attachment point and slowly brought
to the edge of the nanomembrane using the microscope stage.

Figure 1.Series of video frames showing attachment of a functionalized silica bead to a silicon
nanomembrane. The bead is false colored to improve image clarity. (a) The silica bead is trapped in three dimensions with a power of approximately 285 mW
(shown immediately before the bead enters the trap). (b) The nanomembrane is moved toward the trapped bead using the microscope stage. (c) The bead is positioned immediately next to the nanomembrane and a bond forms, linking
the two materials. (d) The nanomembrane rotates slightly in the plane into a preferred position with respect
to the optical trap. The dimensions of the silicon nanomembrane are 50 × 50-μm, 220
nm thick.

As the microsphere is brought into the immediate vicinity of the desired location
on the nanomembrane, a strong van der Waals attraction causes immediate attachment.
The bead attaches to the side of the nanomembrane, as evidenced by both objects remaining
in the same focal plane and the final location of the bead at the very edge. A thin
layer of silicon dioxide readily forms on silicon in the presence of water or alcohols
at room temperature [27]. The oxygen-terminated surface of the nanomembrane reacts at multiple sites with
the hydrogen from the amine-functionalized bead. The resulting bond, shown in Figure
2 at a single attachment site for clarity, has a relatively weak energy of around 20
kJ/mol per bond [28] but is strong enough to allow the simultaneous manipulation of the microsphere and
membrane without detachment. The multiple bonds are also strong enough to rotate the
nanomembrane into an equilibrium position immediately upon attachment while the bead
remains firmly fixed in the trap, also shown in the figure.

Figure 2.Depiction of the bond that forms between the bead and the silicon nanomembrane. A single bond is depicted for clarity. The bead surface is functionalized with -NH2
and the Si surface is oxidized.

Once attached, the bead functions as a nanoscale trailer hitch, facilitating manipulation
of the bead together with the nanomembrane. Using a single optical trap and the motorized
stage, the nanomembrane can be translated laterally over millimeter distances with
laser powers on the order of 200 to 700 mW as measured at the sample. Higher powers
allow larger gradient forces and permit the trapped bead to be moved at greater velocities.

At powers higher than approximately 700 mW, we observe a break in the bond between
the nanomembrane and microsphere, and the two structures become decoupled. Detachment
is observed only if the laser power is greater than this minimum, and we reason that
this results from laser heating in the immediate vicinity of the bead-silicon nanomembrane
attachment site, weakening the coupling moiety, but we cannot at this stage determine
where the break occurs. Most importantly, this decoupling process does not damage
the nanomembrane and we can reattach the membrane to this or a new bead. The reversibility
of our procedure is very encouraging for future work in nanomembrane assembly using
optical tweezers.

In the above range of trapping powers, the nanomembrane can be transported using the
optical tweezers/motorized-stage system at velocities reaching 200 μm/s. Above this
limit, the microsphere and nanomembrane tandem fall out of the optical trap. We observe
a similar maximum velocity when laterally translating identical beads without attachment
to a nanomembrane, suggesting that the hydrodynamic drag resulting from the unique
shape of the nanomembranes is minimal. An order-of-magnitude estimate of the fluid
drag force on the planar nanomembrane is calculated from

FD=CDV22ρA,(1)

where CD is the drag coefficient for a rectangular sheet as a function of area and Reynolds
number, V is the velocity, ρ is the density of the solution, and A is the surface area of the nanomembrane [29,30]. The resulting drag force at maximum velocity for the nanomembranes used in this
work is approximately 0.5 pN, two orders of magnitude lower than the approximately
20 pN calculated for a 10-μm microsphere using Stokes's law, FD = 6πμRV [30]. We expect that this magnitude difference allows for lateral translation of nanomembranes
having much larger surface areas, with minimal effect on motion at similar high velocities.

Finer-scale, more precise manipulation can be achieved using either diffracted traps
from holographic optical tweezers or a steering mirror placed at a conjugate plane
with respect to the objective lens. Figure 3 shows a trapped membrane being transported along a circular path that was predefined
using the control software for the spatial light modulator. This motion occurs approximately
50 μm above the bottom of the sample well. Holographic optical tweezers offer exceptional
control over individual trap positioning and velocity. Nanometer level manipulation
at Angstrom-scale resolutions has been demonstrated for single optical traps [31], and this precision can be extended to multiple traps and multiple structures [32,33]. The benefits of this level of control are offset in part by the limited range of
positioning that can be achieved within a single field of view of the ×20 microscope
objective, approximately 350 × 450 μm. In comparison, the range afforded by our motorized
microscope stage is several centimeters with 10-nm resolution and motion is limited
only by the lateral dimensions of the sample well. Combining holographic optical tweezers
with a motorized microscope stage offers unparalleled nanomembrane manipulation abilities
over a large range of motion.

Figure 3.Extracted video frames showing XY translation of an optically trapped silicon nanomembrane. Frames (a) through (f) show the trapped membrane moved along a circular path in a single imaging plane. The
path was defined using a custom built LabVIEW program to control the output of a spatial
light modulator. The trapped membrane was translated along the path using discrete
angular steps of 2.5°. When moving in a circular fashion, we see rotation of the nanomembrane
around the trapped bead, stemming from the step-by-step directional change of fluid
drag forces acting on the nanomembrane. Similar rotation is not observed when moving
the nanomembrane in a straight line over long distances.

We investigated other methods of membrane manipulation using holographic optical trapping
abilities to demonstrate the versatility of this approach. We found that the in-plane
orientation of a single nanomembrane can be controlled about a single point of rotation,
by harnessing the deleterious scattering force that otherwise precludes direct manipulation
of membranes without an attached bead. Figure 4 shows this rotation. The two-part structure is held in position using a stationary
optical trap that secures the attached microsphere in place while a second dynamically
generated holographic optical trap is used to direct and rotate the object in place.
Once the second optical trap is removed, the membrane remains in its configured position
barring heavy flow in the immediate vicinity. Thus the trapped microsphere functions
as a kind of nanoscale hinge.

Figure 4.Extracted video frames showing laser actuated rotation of an optically trapped silicon
nanomembrane. The silicon membrane is held in place using a single static trap directed at the
attached functionalized bead. Frames (a) through (c) show membrane rotation about this coordinate using a second dynamic optical trap.
The second trap does not directly interact with the nanomembrane but instead uses
the optical scattering force to direct its motion. This indirect approach prevents
damage to the strongly scattering silicon object.

Conclusions

In summary, we have used a functionalized-bead handle technique to translate and rotate
high-lateral-size-to-thickness-aspect-ratio planar silicon nanomembranes in solution
with holographic optical tweezers. The handle technique enables non-contact optical
trapping of two-dimensional planar objects that could not otherwise be manipulated
directly. Our approach permits individual nanomembrane positioning and transfer with
unprecedented lateral control. The use of microspheres allows motion with well-documented
nanometer-scale precision [14], while employing holographic optical trapping facilitates computer control of trajectories
and enhanced positioning accuracy. We also demonstrated reversible attachment and
detachment of handle beads without cutting or damaging the silicon material. We expect
that this trapping method can be extended to manipulating silicon nanomembranes having
larger lateral dimensions and differing thicknesses, in addition to the directed assembly
of various other shapes and material compositions of planar objects with exceedingly
small thickness [34-37]. The ability to tune the bond strength for membranes having different surface terminations
may provide a future path for more selective, simultaneous manipulation of a variety
of different planar materials. The successful use of holographic optical tweezers
demonstrated here could be expanded to include massively parallel control over multiple
nanomembranes, thus making heterostructure stacking and assembly a realizable goal.
This simple proof of concept could eventually enable more advanced non-contact nanofabrication
using nanomembranes as building blocks for two- and three-dimensional optical and
electronic devices.

Methods

Fabrication of Si nanomembranes

The silicon nanomembranes, 220 nm thick, were fabricated from the template (outermost
crystalline silicon) layer of commercially procured silicon-on-insulator (SOI) wafers
(SOITEC S.A., Bernin, France) [38,39]. The wafers were cleaned with acetone, methyl alcohol, and isopropyl alcohol prior
to patterning with electron beam lithography to define the square boundaries of each
nanomembrane. Reactive ion etching was employed to etch the template silicon layer
along these boundaries followed by a wet etch in 49% hydrofluoric acid for 4 h to
dissolve the underlying SiO2 layer. The wet etch causes the patterned, thin silicon membranes to release and settle on
the silicon handle wafer, the bottom layer of the SOI. The patterned membranes were
then removed from the handle wafer by immersion in isopropyl alcohol and re-suspended
in de-ionized water via a solvent exchange, where they remained stably dispersed for
several weeks.

Experimental system

The holographic optical trapping system consists of a linearly polarized IPG Photonics
YLR-10-1064-LP (Oxford, MA, USA) Ytterbium fiber laser operating at 1,067 nm with
a maximum output power of 10 W. The laser is reflected off a Hamamatsu Photonics LCOS-SLM
X10468 (Hamamatsu City, Japan) spatial light modulator (SLM), aligned through beam
expanding optics, and finally directed into an Olympus IX71 inverted microscope (Olympus
America, Inc., Center Valley, PA, USA), as shown in Figure 5. The beam diameter is expanded to slightly overfill the back aperture of the Olympus
objective lens used for all experiments [26], ×20, numerical aperture (NA) = 0.5. Holographic optical traps were generated and
dynamically controlled using a customized LabVIEW program adapted from a freeware
SLM control code available from the University of Glasgow [40,41]. This interface gives us full three-dimensional spatial (translation and rotation)
as well as temporal control over as many as 100 generated traps. Live power measurements
were made continuously at the location of the beam dump, as shown in Figure 5, with more accurate measurements periodically made after the objective lens. All
imaging was done using bright-field microscopy with a Lumenera Infinity 2 (Ottawa,
Canada) digital microscope camera.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

SMO and JRSP performed the translation and manipulation experiments. RBJ and FSF fabricated
the silicon membranes used in these studies. SMO, JRSP, and RJK participated in the
design of the experiment; RJK and MGL supervised the work and prepared the manuscript.
All authors read and approved the final manuscript.

Acknowledgements

This work was supported primarily by the Wisconsin Alumni Research Foundation (WARF)
and the Air Force Office of Scientific Research, Grant# FA9550-08-1-0337. SMO was
supported by an Herb Fellowship from the Materials Science Program at UW-Madison.
Facilities support by NSF, via the UW MRSEC, is acknowledged.